Recently, automotive lamps that use, as a light source, a semiconductor light emitting device such as an LED (light emitting diode) or the like, have been known. When a semiconductor light emitting device is used as a light source for an automotive lamp, the level of light intensity required of the automotive lamp must be satisfied by a maximum use of the light emission from the semiconductor light emitting device.
Generally, a semiconductor light emitting device produces more heat for larger current which is supplied to obtain a greater output. And this correspondingly lowers the luminance efficiency of the semiconductor light emitting device as it gets hotter due to the heating. Thus, there have been various heat radiation structures known for automotive lamps in order to radiate heat from the semiconductor light emitting device efficiently.
For example, in Japanese Patent Application Publication No. 2007-35335, an automotive lamp including a housing whose front end is open, a front lens closing the front end opening portion, a light source unit having a semiconductor light emitting device, a heatsink in contact with the light source unit, and a windmill fan, is disclosed. In the automotive lamp, the windmill fan is rotated by the wind occurring while an automobile is moving and an airflow occurring due to the rotation of the windmill fan flows near the heatsink to cool the heatsink. Thereby, the efficiency in radiating the heat from the semiconductor light emitting device is improved.
A light radiated by a semiconductor light emitting device hardly produces so-called radiation heat effect. Therefore, there is a problem that, when a light from a semiconductor light emitting device is radiated forwards through a translucent cover that covers a front end opening portion of a lamp body, the translucent cover is hardly warmed, and accordingly snow or ice adhered to the outer surface of the translucent cover hardly melts.
To solve the problem, it can be considered that a heat source such as a heater or the like is provided to prevent the adhesion of snow or ice to the outer surface of the translucent cover. However, this measure is not preferable because installation of a heater increases a production cost and needs power for increasing the temperature of the heater. On the other hand, it can be considered that the heat produced by the semiconductor light emitting device is used for preventing the adhesion of snow or ice to the translucent cover. This measure is preferable in terms of power saving and cost.
In the automotive lamp disclosed in the Japanese Patent Application Publication No. 2007-35335, the front lens is slightly warmed because the heat in the heatsink is radiated into the whole air inside the housing, and thereby the effect of melting the snow or ice on the front lens can be obtained to some extent. However, the effect is not sufficient and there is room for improvement.